Wireless automatic meter reading systems are well known. Typically, each utility meter is provided with a battery-powered encoder that collects meter readings and periodically transmits those readings over a wireless network to a central station. The power limitations imposed by the need for the encoder to be battery powered and by regulations governing radio transmissions effectively prevent direct radio transmissions to the central station. Instead, wireless meter reading systems typically utilize a layered network of overlapping intermediate receiving stations that receive transmissions from a group of meter encoders and forward those messages on to the next higher layer in the network as described, for example, in U.S. Pat. No. 5,056,107. These types of layered wireless transmission networks allow for the use of lower power, unlicensed wireless transmitters in the thousands of encoder transmitters that must be deployed as part of a utility meter reading system for a large metropolitan area.
In 1985, as an attempt to stimulate the production and use of wireless network products, the FCC modified Part 15 of the radio spectrum regulation, which governs unlicensed devices. The modification authorized wireless network products to operate in the industrial, scientific, and medical (ISM) bands using spread spectrum modulation. The ISM frequencies that may be used include 902 to 928 MHz, 2.4 to 2.4835 GHz, and 5.725 to 5.850 GHz. The FCC allows users to operate wireless products, such as utility metering systems, without obtaining FCC licenses if the products meet certain requirements. This deregulation of the frequency spectrum eliminates the need for the user organizations to perform cost and time-consuming frequency planning to coordinate radio installations that will avoid interference with existing radio systems.
Spread spectrum modulators use one of two methods to spread the signal over a wider area. The first method is that of direct sequence spread spectrum, or DSSS, while the second is frequency hopping spread spectrum, or FHSS. DSSS combines a data signal at the sending station with a higher data rate bit sequence, which many refer to as a chipping code (also known as a processing gain). A high processing gain increases the signals resistance to interference. FHSS, on the other hand, relies on the distribution of a data signal randomly hopped across a number of defined frequency channels to avoid interference. While DSSS has higher potential data transmission rates than FHSS, DSSS is much more costly than FHSS and has higher power consumption.
U.S. Pat. No. 5,661,750 describes a DSSS system for utility metering that is designed to utilize a high power transmitter and still meet the requirements of FCC Part 15.247. Specifically, in this system, the transmitter utilizes a modulator to modulate the transmission signal with a pseudo-random pattern to spread the signal across a broader bandwidth than the original signal and uses a second modulator to modulate a preamble of the signal with a phase reversal pattern. The phase reversal pattern increases the number of spectrum lines produced by the transmitter and thereby decreases the power density of the broadcast signal, which for DSSS is +8 dBm in any three KHz bandwidth. However, while the phase reversal pattern addresses the low power density requirement, it does not address of the increased cost associated with DSSS—in fact, the addition of a phase reversal modulator likely adds to the cost of the transmitter. Further, it does not address that the DSSS receiver is still significantly vulnerable to noise, and it does not address the issue that only time, rather than time and frequency, may be used for signal collision avoidance; features which are significant and important in a utility sub-metering application.
FHSS operates by taking the data signal and modulating it with a carrier signal that hops from frequency to frequency as a function of time over a wide band of frequencies. With FHSS, the carrier frequency changes periodically. The frequency hopping technique reduces interference because an interfering signal from a narrowband system will only affect the spread spectrum signal if both are transmitting at the same frequency and at the same time. Thus, the aggregate interference will be very low, resulting in little or no bit errors.
A hopping code determines the frequencies the radio will transmit and in which order. To properly receive the signal, the receiver must be set to the same hopping code and listen to the incoming signal at the right time and correct frequency. If the radio encounters interference on one frequency, then the radio will retransmit the signal on a subsequent hop on another frequency. Because of the nature of its modulation technique, FHSS can achieve up to 2 Mbps data rates. It is possible to have operating radios use FHSS within the same frequency band and not interfere, assuming they each use a different hopping pattern.
U.S. Pat. Nos. 5,430,759, 5,499,266, 5,546,422, 5,712,867 and 5,870,426 describe a FHSS system for a paging network to provide low power communications to mobile pagers over an extended coverage area. Although utility metering is identified in these patents as a potential application for the paging network, there are important differences between paging systems and utility meter reading systems that restrict the ability to successfully utilize paging network FHSS technology in a utility meter reading application. Unlike paging systems in which the pager is mobile, the utility meter encoder transmitter is fixed in a single location and reception coverage areas are effectively dictated by the antenna pattern available from that location. While two-way paging system can provide limited communication from a pager to the network, the paging system is not designed to handle continuous periodic reporting of large amounts of data by a large number of units concentrated in a relatively small area.
One of the problems with meter reading applications in the context of wireless radio networks is the potential for collisions between transmissions of a large number of units concentrated in a relatively small area. This problem is particularly acute, for example, in the context of sub-metering applications which involve the allocation of utility usage readings over a large number of units in an apartment, high rise, office building or other dwelling were multiple utility accounts may be located in the same building or in the same building complex. Sub-metering applications also tend to present severe challenges in terms of installation and operation due to structures limiting or blocking effective antenna coverage.
One meter reading system which has been developed for the sub-metering application is the Inovonics Tap Watch® system. In this system, the end point encoder transmitters attached to each utility meter utilize a low power FHSS transmitter having less than 0.5 mW of power and operating under FCC Part 15.249. A network of intermediate repeaters receive the low power FHSS transmissions from the end point transmitters and convert these transmission to DSSS transmissions that are retransmitted by high power transmitter operating under FCC Part 15.247 to base stations for collection and processing. While this approach allows for the use of lower cost end point encoder transmitters, it increases the costs of the intermediate repeaters. Moveover, because the end point encoder transmitters are low power, their transmission range is limited and more intermediate repeaters are required for effective coverage in a sub-metering utility application, for example, thereby further increasing the overall costs of the system.
In view of the above, there is a need for a utility meter reading system that is particularly suited to utility sub-metering, that complies with Part 15.247 of the FCC rules governing spread spectrum devices and that enables a lower overall system while allowing for use of long life battery-operated end-point transmitters and intermediate repeaters, and that also enables improved signal collision avoidance.